1. Field of the Invention
This invention relates to an improved apparatus and method useful to produce sodium citrate. More particularly; the invention is an apparatus and related method to electrodialytically treat an impure aqueous solution of citric acid to produce substantially pure sodium citrate.
2. Description of Related Art
A review of citric acid and its production is presented in Kirk-Othmer, Encyclopedia of Chemical Technology, Second Edition, Volume 5, pages 524 to 541, John Wiley and Sons, Inc. (1964). Citric acid also known as .beta.-hydroxy- tricarboxylic acid or 2-hydroxy-1,2,3-propane tricarboxylic acid.
Citric acid is produced by mycological fermentation of carbohydrates. The fermentation process yields citric acid and the balance of the fermented solution which includes a variety of impurities. In order to separate the citric acid from the impurities the fermentation solutions are filtrated to remove the mycelia, followed by either precipitation of the calcium salt or direct crystallization on concentration of the filtrate. When citric acid is recovered by calcium salt precipitation, the quantity of citric acid in the filtrate after mycelium removal is determined and the amount of calcium hydroxide needed to neutralize the solution is calculated.
The calcium citrate is transferred to an acidification tank. The citrate is suspended in the wash water, from a previous calcium sulfate filtration, and concentrated sulfuric acid is added simultaneously to give a slight excess of sulfuric acid at the end of the batch. The calcium sulfate is filtered off and washed using a conventional industrial filter.
The dilute citric acid solution is purified by de-colorization and de-mineralization. The de-colorization step involves treating of the solution with activated carbon, followed by a polishing filtration. The sparkling clear solution is then passed successively through a cation exchange resin bed and an anion exchange bed.
The de-mineralized citric acid is evaporated in a circulating vacuum pan granulator or in a circulating evaporator-crystallizer. Where sodium citrate is the desired product the citric acid is reacted with sodium hydroxide to yield sodium citrate.
The fermentation products yield by-products which have had little commercial value. They consist of a filter cake obtained from purification of syrup used as raw material, the mycelium filter cake, the filtrate from calcium citrate filtration which contains residual sugars, and hydrated calcium sulfate. The calcium sulfate is of a quality suitable to use as an excipient in pharmacology. Ergosterol has been obtained commercially from the mycelium of the surface fermentation process.
Sodium salts of citric acid are commercially useful. Two sodium citrate hydrates include Na.sub.3 C.sub.6 H.sub.5 O.sub.7.2H.sub.2 O and Na.sub.3 C.sub.6 H.sub.5 O.sub.7.51/2H.sub.2 O. Sodium citrate is used in foods as a buffering agent in conjunction with citric acid and for accurate control of pH in the manufacture of jams, jellies and preserves. It is also used as a stabilizer and emulsifier in processed cheese Sodium citrate additionally has pharmaceutical applications.
Electrodialysis uses direct current as a means to cause the movement of ions in solutions. Electrodialysis processes are well known in the art and are typically carried out in a stack arrangement comprising a plurality of flat sheet membranes. The stack consists of electrodes (anode and cathode) at either end and a series of membranes and gaskets which are open in the middle to form a multiplicity of compartments separated by the membranes. Usually, a separate solution is supplied to the compartments containing the electrodes. Special membranes may be placed next to the electrode containing compartments in order to prevent mixing of the process streams with the electrode streams. The majority of the stack between the electrode compartments comprises a repeating series of units of different membranes with solution compartments between adjacent membranes. This repeating unit is called the unit cell, or simply, a cell. Solution is typically supplied to the compartments by internal manifolds formed as part of the gaskets or by a combination of internal and external manifolds. The stacks can include more than one type of unit cell. Streams may be fed from one stack to another in order to optimize process efficiency. Usually the change in composition of a stream after one pass through the stack is relatively small and the solutions can be recycled by being pumped to and from recycle tanks. Addition of fresh solution to and withdrawal of product from the recycle loop can be made either continuously or periodically in order to control the concentration of products in a desired range.
Treatment of aqueous salt streams by electrodialysis to form acid and/or base from the salt is known. The aqueous salt stream is fed to an electrodialytic water splitting apparatus which comprises an electrodialysis stack and a means for electrodialytically splitting water. A useful apparatus is disclosed in U.S. Pat. No. 4,740,281. A useful means to split water to hydrogen ions (H.sup.+) and hydroxyl ions (OH.sup.-) is a bipolar membrane such as disclosed in U.S. Pat. No. 4,766,161. The bipolar membrane is comprised of anion selective and cation selective layers of ion exchange material. In order for the membrane to function as a water splitter, the layers must be arranged so that the anion layer of each membrane faces the anode. A direct current passed through the membrane in this configuration will cause water splitting with hydroxyl ions being produced on the anode side and a corresponding number of hydrogen ions being produced on the cathode side of the membrane.
Electrodialytic water-splitting in a two-compartment cell has been disclosed, for example, in U.S. Pat. No. 4,391,680 relating to the generation of strongly acidified sodium chloride and aqueous sodium hydroxide from aqueous sodium chloride. U.S. Pat. No. 4,608,141 discloses a multi-chamber two-compartment electrodialytic water splitter and a method for using the same for basification of aqueous soluble salts. U.S. Pat. No. 4,536,269 disclose a multi-chamber two-compartment electrodialytic water splitter and a method for using the same for acidification of aqueous soluble salts. Three-compartment electrodialytic water splitters are disclosed to be comprised of alternating bipolar, anion and cation exchange membranes thereby forming alternating acid, salt and base compartments (B). U.S. Ser. No. 135,562 discloses three-compartment electrodialytic water splitters. U.S. Pat. No. 4,740,281 discloses the recovery of acids from materials comprising acid and salt using an electrodialysis apparatus to concentrate the acid followed by the use of an electrodialytic three-compartment water splitter to separate the acid from the salt.